The present invention relates to a material comprised of particles of a highly porous, low density material in which the pore chambers have been at least partially evacuated and refilled with a gas having a low thermal conductivity, and the obtained particles are encapsulated with a protective and durable coating. The invention also relates to such particles, compositions and articles comprising such particles, and methods for producing the particles. The material of the invention is particularly useful as durable thermal insulating material.
One known group of highly porous, low-density material which has very low thermal conductivity consists of the materials commonly referred to as “aerogels” or “xerogels”, which terms will be used interchangeably in the description of the present invention. In its conventional meaning, the term “aerogel” is used to describe a material obtained by drying a wet sol-gel at temperatures above the critical temperature and at pressures above the critical pressure. Under such conditions, the removal of the gel liquid, for example, water, from the sol-gel results in a porous structure without damaging the structure of the gel, so that a high porosity is obtained. Traditionally, the product obtained by drying at conditions below supercritical conditions is known as a “xerogel”, which has a lower porosity, with at least some of the pore structure being damaged during the drying process. Since the process of drying under supercritical conditions is very energy intensive and costly, attempts have been made to produce xerogels which approximate the properties of aerogels. For example, U.S. Pat. No. 5,565,142 describes “an extremely porous xerogel dried at vacuum-to-below supercritical pressures but having the properties of aerogels which are typically dried at supercritical pressures. This is done by reacting the internal pore surface of the wet gel with organic substances in order to change the contact angle of the fluid meniscus in the pores during drying.”
Silica aerogels were the first extensively studied aerogels. However, aerogels and xerogels may be made with a wide range of chemical compositions. In addition to inorganic aerogels other than silica aerogels, there are organic aerogels prepared from organic polymers and sometimes called “carbon aerogels.”
Aerogels and xerogels can also be surface treated to alter their properties. For example silica aerogel can be made less hydrophilic by converting the surface —OH groups into —OR groups (wherein R is an aliphatic group). U.S. Pat. No. 6,806,299, the content of which is incorporated herein by reference in its entirety, discloses the preparation of hydrophobic organic aerogels.
Aerogels are known to have excellent thermal insulation properties, and xerogels having a porosity and pore structure approximating those of aerogels are also good insulators. Aerogels and xerogels have been the subject of scientific and commercial investigation for use as the thermally insulating component of a variety of thermal barriers and in a variety of applications. Examples of current commercially available aerogel forms include fine particles, beads, or slabs at ambient air pressure. However, the optimal thermal resistance values of aerogels are obtained when they are in a vacuum. Aerogels in the form of fine particles, beads, chunks, blocks, or slabs have been vacuum sealed in plastic wrap or containers, for example, as described in U.S. Pat. No. 6,132,837. These “shrink wrapped” or vacuum-sealed forms of aerogel insulating material are relatively large, as compared to the individual particles. They are not as versatile in that they cannot be incorporated or blended into different medium in the same manner as the particles. These pieces are not readily usable in the construction business, in industrial settings, or for installation by consumers.
The unwrapped forms of aerogel insulating material described above suffer from a lack of durability, and cannot be used in harsh environments or in the presence of abrasive materials. They also suffer from absorption into the aerogel pores of moisture, oils, etc., with a corresponding loss of thermal resistance due to increased overall density and thermal conductivity. The shrink-wrapped form of aerogel insulating material experiences loss of vacuum over time, with a corresponding loss of thermal resistance.